36,437 research outputs found
Energetics of lipid bilayers with applications to deformations induced by inclusions
A new energy for the description of large deformations of lipid bilayers is formulated with mathematical rigor. This energy is derived by considering the smectic A liquid crystalline nature of lipid bilayers and the coupling between the deformations of the layers and their constituent lipid molecules. Analogies between smectic A liquid crystals, with an infinite number of layers, and lipid bilayers, with a finite number of layers, are further discussed. The novelty of the energy density is demonstrated by studying the large deformations of planar lipid bilayers induced by cylindrical inclusions. The results of this study are directly compared with the results obtained using May's theoretical framework [May, Eur. Biophys. J., 2000, 29, 17–28] in which small deformations are assumed. As expected, the proposed energy density predicts larger distortions of the lipid molecules and deformations of the lipid bilayers close to an inclusion
Purity or pragmatism? : Reflecting on the use of systematic review methodology in development
Systematic review methodology pioneered in health care has been increasingly applied to development questions of importance in lower- and middle-income countries. This paper reports one such review on the topic of microfinance in sub-Saharan Africa and reflects on the number of pragmatic methodological compromises made when applying the method to a new field. These compromises relate to multidisciplinary teamwork, application of regional filters, drawing on evidence from additional study types and exploring mechanisms for change through the development and testing of a causal pathway. The paper concludes that a pragmatic rigorous approach to systematically reviewing evidence of effectiveness is needed for international development
The Power Spectrum for a Multi-Component Inflaton to Second-Order Corrections in the Slow-Roll Expansion
We derive the power spectrum of the density perturbations
produced during inflation up to second-order corrections in the standard
slow-roll approximation for an inflaton with more than one degree of freedom.
We also present the spectral index up to first-order corrections including
previously missing terms, and the running to leading order.Comment: 11 pages, no figure. KAIST-TH 2002/0
Naked and Thunderbolt Singularities in Black Hole Evaporation
If an evaporating black hole does not settle down to a non radiating remnant,
a description by a semi classical Lorentz metric must contain either a naked
singularity or what we call a thunderbolt, a singularity that spreads out to
infinity on a spacelike or null path. We investigate this question in the
context of various two dimensional models that have been proposed. We find that
if the semi classical equations have an extra symmetry that make them solvable
in closed form, they seem to predict naked singularities but numerical
calculations indicate that more general semi classical equations, such as the
original CGHS ones give rise to thunderbolts. We therefore expect that the semi
classical approximation in four dimensions will lead to thunderbolts. We
interpret the prediction of thunderbolts as indicating that the semi classical
approximation breaks down at the end point of black hole evaporation, and we
would expect that a full quantum treatment would replace the thunderbolt with a
burst of high energy particles. The energy in such a burst would be too small
to account for the observed gamma ray bursts.Comment: 21 pages (10 diagrams available on request
The Measure Problem in Cosmology
The Hamiltonian structure of general relativity provides a natural canonical
measure on the space of all classical universes, i.e., the multiverse. We
review this construction and show how one can visualize the measure in terms of
a "magnetic flux" of solutions through phase space. Previous studies identified
a divergence in the measure, which we observe to be due to the dilatation
invariance of flat FRW universes. We show that the divergence is removed if we
identify universes which are so flat they cannot be observationally
distinguished. The resulting measure is independent of time and of the choice
of coordinates on the space of fields. We further show that, for some
quantities of interest, the measure is very insensitive to the details of how
the identification is made. One such quantity is the probability of inflation
in simple scalar field models. We find that, according to our implementation of
the canonical measure, the probability for N e-folds of inflation in
single-field, slow-roll models is suppressed by of order exp(-3N) and we
discuss the implications of this result.Comment: 22 pages, 6 figures. Revised version with clarifying remarks on
meaning of adopted measure, extra references and minor typographical
correction
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